1. Field of the Invention
The present invention relates to a differential transmission lines and, in particular, to a differential transmission line that transmits an analog high-frequency signal or a digital signal in a microwave band, a sub-millimeter wave band or a millimeter wave band.
2. Description of the Related Art
A differential signal transmission system, which has less radiation and is also robust to noises as compared with the single-ended signal transmission system that has been conventionally used, has been increasingly used for high-speed signal transmissions.
FIG. 12 is a top view of a prior art differential transmission line. FIG. 13 is a longitudinal sectional view along the line C-C′ of the differential transmission line of FIG. 12 showing an electric field vector Ee in an odd mode. FIG. 14 is a longitudinal sectional view along the line C-C′ of the differential transmission line of FIG. 12 showing an electric field vector Eo in an even mode.
Referring to FIGS. 12 to 14, a grounding conductor 11 (FIGS. 13 and 14) is formed on the back surface of a dielectric substrate 10, and strip-shaped signal conductors 2a and 2b in parallel to each other are formed on the front surface of the dielectric substrate 10. Differential high-frequency signals of signs opposite to each other are applied to the two signal conductors 2a and 2b, and the line functions as a differential transmission line. That is, a first microstrip line 20a (FIG. 12) is constituted by including the signal conductor 2a and the grounding conductor 11 sandwiching the dielectric substrate 10, and a microstrip line 20b (FIG. 12) is constituted by including the signal conductor 2b and the grounding conductor 11 sandwiching the dielectric substrate 10. In this case, the differential transmission line is constituted by including a pair of the microstrip lines 20a and 20b. 
If the two microstrip lines 20a and 20b are adjacently placed to be parallel to each other so as to be electromagnetically coupled with each other as shown in FIGS. 12 to 14, two modes of the even mode in which signals in an identical direction are transmitted through the two microstrip lines 20a and 20b and the odd mode in which signals in opposite directions are transmitted are generated. In the differential transmission line, a differential signal is transmitted by utilizing the odd mode.
The electric field vector Ee in the odd mode is schematically indicated by the arrow of FIG. 13, and the direction of the electric field vector Eo in the even mode is schematically indicated by the arrow of FIG. 14. In the odd mode, as shown in FIG. 13, the electric field vector Ee is directed from one signal conductor 2a toward the other signal conductor 2b, and the magnitude of the electric field vector directed from the signal conductor 2a to the grounding conductor 11 is small. That is, a virtual ground plane is formed on the symmetry plane of the two signal conductors 2a and 2b according to the differential transmission in the odd mode.
In designing a differential transmission line, a circuit design such that the inputted differential signal is not converted into a common-mode signal is indispensable. For example, in order for two signals inputted with opposite phases and an equal amplitude to keep the opposite-phase equal-amplitude relation, it is necessary to keep a circuit symmetry of the two microstrip lines 20a and 20b, through which the respective signals are transmitted. That is, the two microstrip lines 20a and 20b that constitute the differential transmission line need to be a pair of transmission lines that have identical amplitude characteristics and phase characteristics. However, at the bends of the differential transmission line (i.e., bend regions of the two microstrip lines 20a and 20b), unnecessary mode conversion from the differential signal to the common-mode signal easily occurs.
The Patent Document 1 of the first prior art discloses a measure for removing the unnecessary common-mode signal that has been disadvantageously superimposed on the differential transmission line. FIG. 15 is a top view showing differential transmission lines 20A and 20B of the first prior art. The construction of the differential transmission lines 20A and 20B disclosed in the Patent Document 1 is described below with reference to FIG. 15.
Referring to FIG. 15, a plurality of slots 21 are formed at a grounding conductor (not shown, but referring to a ground conductor formed on the back surface of the dielectric substrate 10) just under the differential transmission lines 20A and 20B. The slots 21 extend in a direction orthogonal to the transmission direction 25 of a differential signal. By adopting the construction as described above, impedance to the common-mode signal is selectively increased, and the common-mode signal is reflected. According to the differential-mode transmission, a virtual high-frequency ground plane is formed between a pair of signal conductors 2a and 2b that constitute the differential transmission line 20A, and therefore, an influence on the transmission characteristic is small even if the plurality of slots 21 are formed on the grounding conductor. Therefore, at the differential transmission lines 20A and 20B of the first prior art described in the Patent Document 1, the transmission characteristic in the differential mode is not negatively influenced, and it is possible to reduce a common-mode signal transmission intensity.
The Patent Document 1 further discloses a method for removing the common-mode signal at the bends of the differential transmission line 20B. That is, the Patent Document 1 describes that it is effective for the removal of the common-mode signal to form a slot 23 in a direction orthogonal to the local signal transmission direction 27 also in the case where the differential transmission line 20B has a bend shape as in the case of the linear shape. Moreover, the Non-Patent Document 1 discloses a principle that the common mode can be removed by forming the slots 21 and 23 at the grounding conductor.
The related documents to the present invention are as follows:
Patent Document 1: JP 2004-048750 A;
Non-Patent Document 1: F. Gisin et al., “Routing differential I/O signals across split ground planes at the connector for EMI control”, 2000 IEEE International Symposium on Electromagnetic Compatibility, Vol. 1, pp. 325-327, August 2000;
Non-Patent Document 2: M. Kirschning et al., “Measurement and computer-aided modeling of microstrip discontinuities by an improved resonator method”, 1983 IEEE MTT-S International Micro wave Symposium Digest, Vol. 83, pp. 495-497, May 1983; and
Non-Patent Document 3: A. Weisshaar et al., “Modeling of radial microstrip bends”, 1990 IEEE MTT-S International Microwave Symposium Digest, Vol. 3, pp. 1051-1054, May 1990;
However, according to the prior art described above, the intensity of the common-mode signal transmitted through the differential transmission line can be reduced when the common-mode signal is inputted, whereas there is neither disclosure nor suggestion regarding a reduction in the unnecessary mode conversion intensity with which the common-mode signal is outputted when the differential signal is inputted.
FIG. 16 is a top view showing a differential transmission line 20C according to the Non-Patent Document 2 of a second prior art. The Non-Patent Document 2 discloses that the transmission characteristic is improved by removing the corner 29 of a signal conductor 2 at the bend of the single-ended microstrip line 20C as shown in FIG. 16. In general, a grounding capacitance generated between the signal conductor 2 and the grounding conductor tends to increase at the bend of the microstrip line 20C in comparison with the linear regions. Therefore, when the area of the signal conductor 2 is reduced at the bend, the transmission characteristic is improved. This technique is widely used for the contemporary high-frequency circuit design. As for software or the like to make a layout chart from a circuit diagram, it is often the case where the removal of the corner portion at the bend of the signal conductor is automatically set.
The Non-Patent Document 3 of a third prior art reports the high-frequency characteristic of a line structure exhibiting a satisfactory value as a transmission characteristic in the high-frequency band at the bend of the single-ended microstrip line. Although it is contemplated that the reflection of the transmission signal might occur in the construction of the second prior art, the high-frequency characteristic is improved by assuming the center of curvature at the curve of the transmission line and laying the signal conductor gently curved in the construction of the third prior art. Such a construction is also generally used in the high-frequency circuit to transmit particularly a high-frequency signal.
FIG. 17 is a top view showing a differential transmission line 20D according to a modified example of the first prior art. The bends of the differential transmission line shown in FIG. 17 can be achieved on the basis of the disclosed contents of the first prior art. The line structure of the bends shown in FIG. 17 corresponds to one such that the slot 23 is removed from the line structure of the bend shown in FIG. 15.
FIG. 18 is a top view showing a differential transmission line 20E according to a modified example of the third prior art. It is also possible to achieve the curve of the differential transmission line shown in FIG. 18 on the basis of the disclosed contents of the third prior art. In this case, the center of curvature is assumed at the curve, and two signal conductors 2a and 2b that are arranged gently curved are arranged so as to be parallel to each other.
According to the constructions of the Patent Document 1 and the Non-Patent Document 1, the effect of suppressing the unnecessary mode conversion from the differential signal (i.e., transmission signal in the odd mode) to the common-mode signal (i.e., transmission signal in the even mode) at the bends and asymmetric lines cannot be obtained. Since the unnecessary mode conversion significantly occurs as the transmission frequency increases at the bends of the differential transmission line, satisfactory differential-mode transmission cannot be achieved. Moreover, even if the structures proposed to improve the high-frequency characteristic of single-ended signal transmission by the Non-Patent Documents 2 and 3 are applied to the bends of the differential transmission line, the unnecessary mode conversion cannot be sufficiently suppressed.